1. Field of the Invention
The present invention relates to a method of forming a tubing lined borehole and to a downhole cutting tool. In particular, but not exclusively, the present invention relates to a downhole cutting tool for use in enlarging an existing borehole. The invention also relates to a method of enlarging a borehole.
2. Description of the Related Art
In the oil and gas exploration and production industry, it is a common practice to drill a borehole to a desired depth for recovering well fluids from hydrocarbon-bearing rock formations. Logging procedures are carried out both before and during drilling of the borehole to determine physical characteristics of the rock formations. Typically, the borehole is drilled to a first depth before locating a metal casing of a first outer diameter in the borehole, suspending the casing from a wellhead, and cementing the casing in place. Further logging procedures are then carried out to determine more accurately the physical characteristics of the borehole at depth, and the borehole is then extended to a second depth by drilling a smaller diameter borehole extending from the upper, cased borehole.
This smaller diameter extension is then cased with a smaller diameter casing extending from the wellhead, which is also cemented in place to, inter alia, seal the intersection between the upper, larger casing and the smaller diameter casing. This process is continued until the borehole has been cased and cemented to a desired depth and completion procedures are then carried out to allow recovery of well fluids.
This traditional method of casing a borehole is both time-consuming and costly as it involves locating multiple lengths of casing in the borehole, each extending from the wellhead. This employs long lengths of expensive metal casing and large volumes of cement.
Furthermore, in the event of a problem being encountered during drilling of the borehole, such as drilling fluid being lost into a fractured or highly permeable formation, it is necessary to conduct remedial operations to overcome such problems. This typically involves running an additional length of casing back to the wellhead to isolate the problem formation.
Although this eventuality is allowed for during planning of the well, it is generally undesired and too many such occurrences can have a significant effect upon the final diameter of the borehole and thus the ability to conduct completion procedures.
Much research has been carried out in the industry in an effort to facilitate the creation of mono-bore wells: a mono-bore well is a borehole cased with tubing of a constant internal diameter, to avoid the need to provide multiple overlapping lengths of casing suspended from the wellhead.
To this end, expandable casing, liner and hanger systems are being developed in an effort to achieve a mono-bore well, which will also extend drilling capabilities by increasing the opportunities for use of intermediate and slim profile casing strings. Intermediate strings are used to cover problem areas, such as deteriorated casing, as a form of patch, whilst slim profile strings comprise relatively thin wall tubing which take up less space downhole.
However, various problems have been encountered. For example, to achieve a mono-bore cased borehole, it is necessary to form a “bell end” at the lower end of a casing string, to provide a recess into which the subsequent casing can be nested. This is not possible with existing casing strings because the casing is cemented into the wellbore. Accordingly, a hard cement material is located around the outside of the casing shoe (the last section of the upper or previous casing), which prevents the casing from being formed into a bell end.
The Applicant's International Patent Publication No. WO 02/25056 discloses a liner shoe including a compressible material defining an annular sleeve around an outer surface of a lower end of the shoe. This compressible material prevents cement from surrounding the end of the shoe when the liner is cemented, and allows the end to be subsequently diametrically expanded to form a bell end. However, the liner shoe cannot be used in existing well boreholes cased and cemented as described above.
It is amongst the objects of embodiments of the present invention to obviate or mitigate at least one of the foregoing disadvantages.